Under most circumstances, Exchange infrastructure is spread across multiple physical or virtual servers. Just which sub-functions are installed on the respective servers is defined through server roles. While the previous version required setting up mail boxes of individual users and accepting client connections to be carried out through their separate roles, Mailbox and Client Access, Exchange Server 2016 now contains all of these main functions within the mailbox server. This is where the databases are stored and the client requests are received, making an additional client access server superfluous.
In the current version, the Edge Transport Server maintains its old positon as a second server. Most of the time, this server is separately installed in an upstream perimeter network, the so-called demilitarized zone (DMZ). Through anti-spam and e-mail flow rules, this serves as an additional safeguard for e-mail transfer occurring between local networks and the internet. Following this, classic Exchange architecture consists of an edge transport server and any number of mailbox servers that are organized in a data base availability group (DAG). The load distribution is regulated by a load balancer.
Access to the mailbox server is generally gained through the client program, Outlook, the web app Outlook on the web, or a mobile device. Alternatively, Microsoft Exchange Server functions can also be used via the Linux software, Evolution. The central interface for communicating with Windows client programs is MAPI (Messaging Application Programming Interface). Outlook for Macintosh uses the server interface EWS (Exchange Web services). RPC (Remote Procedure Call) or HTTP (Hypertext Transfer Protocol) are used as transfer protocols.
While client requests from the local network or externally via the internet directly access the mailbox servers of the Exchange architecture through the load balancer, e-mails originating from an external SMTP server first pass the perimeter network with the Edge Transport server before being forwarded to the load balancer and thus to the mailbox servers. The following graphic shows the classic structure of an Exchange architecture (version 2016):
